Electrochemical machining (ECM) and pulsed electrochemical machining (PECM) make it possible to machine workpiece surfaces very precisely and without stress to the material. In this process, a potential differential is built up between a workpiece that serves as the anode and an electrode that serves as the cathode or tool and that is positioned across from the workpiece area that is to be machined, after which material is removed from the workpiece via an electrolyte, for example, a saline solution. Since the electrode usually replicates the desired contour of the workpiece surfaces that are to be machined, the workpiece area can be machined virtually in one step.
Electrochemical machining can be traced back to a Russian development and was already patented back in 1929 at the German Reich Patent Office under number DE 565765. The main difference between the ECM method and the PECM method is that with the PECM method, the electrode executes a pulsed motion in the advancing direction, so that a narrow machining gap filled with the electrolyte is opened or enlarged between the electrode and the workpiece, as a result of which the spent electrolyte can drain away more easily.
A problematic aspect encountered in electrochemical machining, however, is the control of the electrolyte flow. This is why German patent application DE 1515195 A1 proposes an electrode consisting of a plurality of partial electrodes that are electrically insulated from each other and that are actuated or activated as a function of a relative position between the electrode and the workpiece.
By the same token, German patent application DE 10 2007 023 494 A1, particularly in order to form fluid bearings, discloses the approach of providing electric insulation on the surfaces areas of an electrode from which no material is to be removed.
German patent application DE 10 2004 040 217 A1 of the applicant describes an ECM/PECM method for forming a cavity in a workpiece. The workpiece is placed in an electrolyte immersion bath and an energized electrode is sunk into the workpiece. However, it has been found that the resultant side surfaces of the cavity created in the workpiece, that is to say, the workpiece surfaces that are parallel to the advancing direction or the workpiece surfaces that delimit a side gap with the electrode, are formed with inferior surface quality than a bottom surface of the cavity in the workpiece, which requires subsequent machining of the side surfaces of the workpiece, as a result of which disturbances can occur in the transition area leading to the bottom surface of the workpiece.
Even though electrodes with retractable partial electrodes are known, their mechanical implementation is very complicated. It is also a known procedure to machine the cavity in two stages, so that the bottom surface is machined first, and then the side surfaces of the cavity. In this process, the side surfaces are sunk with a special electrode that is moved as orthogonally as possible with respect to the side surfaces. A drawback of this known variant is especially the fact that it has to be carried out with a double set of electrodes and that, when the side surfaces are sunk, a certain amount of etching can occur in the area of the bottom surface.